Carbon nanotubes are well known in technology. They consist, in general, of extremely fine tubular filaments made of pure carbon. Their diameter is a few nanometres and their length may reach several hundreds of nanometres, or approach or even exceed a micrometre.
At present there are three principal methods for synthesising carbon nanotubes: the laser carbon ablation method, the discharge of an electric arc on a graphite electrode and the chemical decomposition of a hydrocarbon in the vapour phase, also called the CCVD method (CCVD being an abbreviation for Catalyst Chemical Vapour Decomposition).
The CCVD synthesis method has the advantage of being inexpensive and of producing nanotubes with satisfactory carbon yields. According to this known method a hydrocarbon (for example, ethylene) is decomposed in the gaseous state at high temperature (of the order of 1000° C.) in the presence of a catalyst and the carbon nanotubes (which may be accompanied by soot and impurities), the catalyst, hydrogen, the hydrocarbon fraction which has not been decomposed and, in some cases, heavier hydrocarbons (for example, ethane if the hydrocarbon subjected to catalytic decomposition is ethylene) and odoriferous constituents are recovered from the synthesis reactor.
With the CCVD method of synthesis it is essential to avoid the presence of air in the synthesis reactor, to prevent the formation of explosive mixtures with the hydrocarbon or hydrogen produced. To this end, in a known implementation of the CCVD method, use is made of a tubular quartz reactor of the fixed-bed type at the centre of which is arranged a quartz receptacle containing a charge of catalyst (Chemical Physics Letters 317 (2000) pages 83 to 89: “Large-scale synthesis of single-wall carbon nanotubes by catalytic chemical vapor deposition (CCVD) method”, J.-F. Colomer et al.). This known implementation of the CCVD method implies the following operations in order of execution:                placing of the installation under inert atmosphere;        opening of the reactor to insert a receptacle containing fresh catalyst;        placing of the installation under inert atmosphere;        feeding of the reactor with hydrocarbon in an inert gas;        placing of the installation under inert atmosphere;        withdrawal of the receptacle with the catalyst and the raw synthesis product from the reactor.        
This known implementation of the CCVD method is discontinuous, which constitutes a disadvantage and is detrimental to the productivity of the synthesis reactor. It has the additional disadvantage of introducing the fresh catalyst into the reactor under inert gas at the high reaction temperature, whereas the catalysts used are deactivated under these conditions. The degree of deactivation of the catalyst is greater in that the time necessary to purge the installation and place it under inert atmosphere, after having introduced the fresh catalyst, is long.
To reduce the degree of deactivation of the catalyst it has been proposed to increase the length of the synthesis reactor in such a way that it has an upstream portion housed in an oven and a downstream portion in ambient air. The hydrocarbon and its carrier gas are introduced into the upstream portion of the reactor and the fresh catalyst is introduced into the downstream portion of the reactor. In this way the reactive gas is cooled in the downstream portion of the reactor and reaches the middle of the reactor at ambient temperature. In this case the reactor can be opened under reactive atmosphere to withdraw the receptacle charged with raw synthesis product and insert another receptacle with the fresh catalyst without excessive risk. This known variant of the CCVD method has the advantage of being faster, since the purgings with inert gas are no longer necessary. However, it does present a non-negligible hazard of explosion since non-negligible quantities of hydrocarbon and hydrogen are brought into contact with ambient air. In addition, partial deactivation of the catalyst upon contact with the reactive gases before reaching the reaction temperature is not avoided. Furthermore, because of the thermal inertia of the quartz receptacle/catalyst assembly, the catalyst requires a non-negligible time to reach the reaction temperature after being introduced into the hot portion of the reactor, further increasing its premature deactivation.